A preferred technique to promote the regeneration of mammalian tissue is accomplished by the separation and isolation of a particular type of tissue to be regenerated from other competing undesirable tissues through the use of a biocompatible barrier material. This concept is known as guided tissue regeneration and was described in an article by J. Gottlow, et al., titled "New Attachment Formation in the Human Periodontium by Guided Tissue Regeneration" (Journal of Clinical Periodontology, 1986; Vol. 13, pp. 604-616). The function of the barrier material is to substantially preclude the movement of tissue cells through the thickness of the material and consequently limit the varieties of cell types at the treatment site. This function is combined with the requirement that the material maintain sufficient space adjacent to the defect so as to allow for the regeneration of the desirable tissue into that space. The preservation of space between the surface of the defect and the desired contours of the subsequently regenerated surface is necessary in order to allow for the regeneration of tissues into that space. Specific periodontal structures which may be regenerated in this fashion are the periodontal ligament, bone and cementum. The barrier material allows propagation of bone and periodontal ligament cells by precluding epithelial cells and gingival connective tissue cells which are believed to propagate at a greater rate. This concept may be useful for other applications where separation of specific cell varieties is desirable such as, for example, nerve repair and nerve guidance applications, bone regeneration and prevention of soft tissue adhesions, particularly those of the peritoneum.
A description of a bioabsorbable tubular device useful for nerve repair is provided in U.S. Pat. No. 4,870,966 to Dellon, et al.
Additionally, it has been proposed that the mechanical stability of the blood clot which forms in the defect space adjacent to the tooth root after periodontal surgery may be important to the regeneration process (Wikesjo et al., 1990; J. Periodontal., Vol. 61, 559-563). Therefore a material which can become infiltrated with blood clot and thus form a connection between the material and the adjacent gingival flap may add to the mechanical stability of the wound.
One commercially available material that provides a cell-barrier for periodontal tissue regeneration is the GORE-TEX Periodontal Material. This polytetrafluoroethylene (PTFE) material serves as a cell-barrier between the gingiva and a periodontal defect and is intended to preserve necessary space between the surface of the defect and the desired contours of the subsequently regenerated surface. The GORE-TEX Periodontal Material is made of porous expanded PTFE having a microstructure of nodes interconnected by fine fibrils. This commercially available material is not of laminar construction and its porosity is generally uniform through the thickness of the material. One portion of the total surface area of the GORE-TEX Periodontal Material has a porous structural surface that becomes infiltrated with blood clot and ingrown with fibrous connective tissue, thereby inhibiting epithelial migration. The remaining portion of the surface area has a cell-barrier structure of low porosity for isolating the overlying gingival connective tissue from the underlying defect. It is not bioabsorbable and is typically removed in a subsequent surgical procedure.
There have been previous attempts to produce suitable surgical barriers from bioabsorbable materials. A 70 micron thick membrane solvent-cast from bioabsorbable polylactic acid, having no inherent porosity or tissue cell permeability, was tested in periodontal applications as a cell-barrier material for exclusion of epithelium and gingival connective tissue during healing (I. Magnusson, et al., "New Attachment Formations Following Controlled Tissue Regeneration Using Biodegradable Membranes", J. Periodontal January, 1988 pp. 1-6). Tests showed some new formation of cementum and bone. Reproductions of this material demonstrated poor surgical handling characteristics due to its thin friable construction and also proved to be difficult to suture because of its brittleness. This material makes no provision for tissue ingrowth on either of its surfaces.
Another material that is commercially available for use in guided tissue regeneration is VICRYL Periodontal Mesh available from Johnson & Johnson. The VICRYL Periodontal Mesh is comprised of woven fibers made from a bioabsorbable copolymer of about 90% glycolide and 10% lactide. Studies have shown that the VICRYL Periodontal Mesh has had some success as a barrier material that provides for tissue regeneration (Fleisher et al., "Regeneration of lost attachment apparatus in the dog using Vicryl Absorbable Mesh", International Journal of Periodontics and Restorative Dentistry 2/1988 pp 45-55). Difficulties with this conventional woven construction include its inferior ability to maintain space adjacent to the defect and its marginal effectiveness as a tissue barrier because of the inherent porosity of the woven structure. This material is a single layer material of woven construction that is intended to both promote tissue ingrowth and simultaneously serve as a tissue barrier. As these are somewhat contradictory objectives for a single layer material of woven construction having a degree of inherent porosity, ingrowth can only be made to occur at the expense of the barrier function. The effectiveness of this material is therefore a compromise between the material's ability to allow for tissue ingrowth and the requirement to simultaneously function as a tissue barrier.
There remains a need for a bioabsorbable material for use as an effective cell-barrier that allows for tissue attachment on at least one surface, adds to the stability of the healing wound through blood infiltration and coagulation into the material, substantially precludes passage of tissue cells through the material, possesses adequate rigidity to ensure preservation of the desired space proximal to the defect and has acceptable surgical handling properties and strength.
Bioabsorbable materials are herein defined as those materials of either synthetic or natural origin which when placed into a living body are degraded through either enzymatic, hydrolytic or other chemical reactions, into byproducts which are either integrated into or expelled from the body.
Cells and tissue are herein defined as mammalian cells and mammalian tissue.